Cast iron pipes, such as ductile cast iron pipes, are often formed on the surface thereof with a corrosion preventive layer for protection of the surface against corrosion. Known types of corrosion preventive coatings include a coating formed by applying a paint to the surface, and a sprayed coating formed by arc spraying a metal material such as a zinc alloy on the surface of the pipe.
Of these types of coatings, the former type in which the coating is formed by mere paint coating has a disadvantage that where a pipeline is installed in a highly corrosive environment, any corrosion, once it occurred, cannot be prevented from progressing further. Where the pipeline is buried in the ground, the coating may be partially scraped off the surface during the work for pipeline placement whereby the pipeline is rendered partially defective so that a local corrosion may readily develop at the defective portion of the pipeline.
In the case of sprayed zinc alloy coating, the coating functions as a sacrifice electrode because zinc exhibits a higher degree of ionization tendency than iron or the material of the pipe. Therefore, improved anti-corrosion effect can be obtained as compared to the case of mere paint coating. However, when the sprayed zinc alloy coating has been completely consumed as the sacrifice electrode, the coating can no longer provide any corrosion preventive effect. Another disadvantage is that since zinc is so soft that the coating, as is the case with the mere paint coating, may be partially scraped off the pipe surface and rendered defective so that the material iron is locally exposed and made susceptible to corrosion. In the case of a surface coating being formed by arc spraying, the coating may be porous so that an oxide scale tends to be formed between the iron material and the coating, resulting in lowered anti-corrosion effect of the coating.